1. Field of the Invention
This invention relates to a biasing scheme for field effect transistors, hereinafter referred to as FETs, and more particularly to a series biasing scheme for a pair of FETs, particularly useful in the design of radio frequency (RF) amplifiers, mixers and oscillators for low current applications.
2. Description of the Prior Art
FETs are frequently employed as the active element in RF amplifiers. Mixers or oscillators may employ FETs as active devices as well. Since silicon devices generally do not operate at the high frequencies required, most microwave devices use N channel junction transistors made of gallium arsenide. It is also theoretically possible to manufacture P channel junction transistors of gallium arsenide. However, since the mobility of holes, which serve as the majority carriers in P channel devices, is much less than the mobility of electrons, which serve as the majority carriers in N channel devices, P channel devices are rarely used.
The FET devices must be turned on, or biased, with DC voltages so that they can linearly amplify an RF input signal. Certain amplifier designs, such as the balanced amplifier and the push-pull amplifier, where there are two identical branches connected so as to operate with specific phase relationships, require that the FETs be in pairs. The balanced amplifier configuration refers to a specific amplified topology, utilizing two matched active elements, that exhibits good input and output return loss characteristics. The push-pull configuration is similar to the balanced amplifier topology, employing 180.degree. rather than 90.degree. couplers. The prior art DC biasing scheme is to bias these pairs in parallel, thus requiring twice the DC current of any single FET.
A close matching of the FET pair is required in order to provide the proper biasing of a pair connected in series. Series biasing guarantees that the current across each of the FETs is equal because they share the same current. The drain to source voltages must also be equal. However, in general, a manufacturer cannot build a large number of JFETs with the same characteristics. The values of the pinch off voltage (Vp) and the saturated current (I.sub.dss) associated with each JFET in a single wafer in which a plurality of JFETs are formed can typically vary by a factor of greater than 2 to 1 over the wafer. Therefore, in a passive biasing scheme, a factory adjusted resistor is needed to bias a given JFET to a given bias point. This adjustment involves time and money.
Roveti, in U.S. Pat. No. 3,656,025, describes a current limiter comprised of a pair of field effect transistors connected in series with a biasing resistor. One of the transistors has a lower cut-off potential than the other, and the one with the higher cut-off potential also has substantially higher resistance and can withstand much higher voltages. Roveti's device does not address the problem of unequal voltage drops across the two FETs. Rather, the operation of his device relies upon uneven voltage drops across the two series-connected field effect transistors.
A biasing arrangement, described by Yokoyama in U.S. Pat. No. 4,037,166, includes two complementary field effect transistors (FETs), connected in series with a constant current source. The gate bias voltages of the FETs are stabilized against fluctuations of the ambient temperature so that the operating points of these FETs will not fluctuate. Therefore, the operation of the amplifier in which they are employed remains unaffected by variations in temperature.
Yokoyama, U.S. Pat. No. 4,238,737, teaches a biasing arrangement comprising a pair of gate biasing resistors for field effect transistors forming a push-pull amplifier, and two constant current supplies having two output terminals for equal amounts of current to the transistors. Each constant current supply can be adjusted manually or automatically for setting a suitable operation point of the transistors and for balancing their bias voltages. The key is to balance the devices, i.e. to balance the gate to source bias voltages and provide the same current through the two current sources (see Yokoyama, column 2, lines 50 to 68). Such a push-pull amplifier employing a 180.degree. hybrid coupler is seldom used in microwave technology. The more commonly used balanced amplifier, utilizing a 90.degree. hybrid coupler, provides better input and output voltage standing wave ratios (VSWRs).
Sasaki, U.S. Pat. No. 4,433,303, discloses a push-pull amplifier circuit employing two field effect transistors of opposite conductivity type to each other which are activated by a common input signal to perform a push-pull operation. Secondary distortion components are cancelled out by impedance elements located between the source of each FET and a common reference potential point (Sasaki, column 2, lines 52 to 55). The apparatus makes use of a complementary symmetry within the circuit, so that whatever distortion is in one part of the circuit is cancelled out in the other part of the circuit. The main goal of Sasaki's invention is to reduce distortion.